CH318OH: Assignment of FIR Laser Lines Optically Pumped in the in-Plane CH3-Rocking Band

✍ Scribed by S.B. Zhao; R.M. Lees

• Publisher: Elsevier Science
• Year: 1994
• Tongue: English
• Weight: 760 KB
• Volume: 168
• Category: Article
• ISSN: 0022-2852
• DOI: 10.1006/jmsp.1994.1260

✦ Synopsis

Over 100 far-infrared (FIR) laser emission lines. with wavelengths ranging from (3410653 \mu \mathrm{m}), have been reported for the (\mathrm{CH}{3}{ }^{18} \mathrm{OH}) isotopomer of methanol optically pumped by (\mathrm{CO}{2}) lasers. Most of these FIR laser lines arise from transitions within the CO-stretching state, due to the good overlap between the CO-stretching band of (\mathrm{CH}{3}{ }^{18} \mathrm{OH}) and the 10.6 and (9.6-\mu \mathrm{m} \mathrm{CO}{2}) laser bands. However, some of the FIR lines are pumped by high-lying (\mathrm{CO}{2}) transitions of the (9.6-\mu \mathrm{m}) band and clearly originate from a different vibrational state. In a detailed analysis of the highresolution Fourier transform infrared spectrum of (\mathrm{CH}{3}{ }^{18} \mathrm{OH}), we have identified the weak inplane (\mathrm{CH}{3})-rocking band lying just above the strong (\mathrm{CO})-stretching band, and have assigned 11 of the FIR laser lines to rocking-state transitions along with an additional 2 lines assigned to COstretching transitions. An important and unexpected tinding was the existence of strong Fermi resonance between the torsionally excited (n=1) state of the rocking mode and the ground (n=0) state of the (\mathrm{OH})-bending mode. which produces significant level shifts and mixing and leads to two of the lasing transition systems. Here, we present our FIR laser assignments together with the spectroscopic evidence used to confirm them through combination-difference loops. The interplay between the FIR laser observations and the spectroscopic analysis has proved beneficial in supporting the assignments and in yielding information for the accurate location of levels of the elusive (\mathrm{CH}{3})-rocking state.